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Batuhan Osman TASKAYA
cpython
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c9fbb72b
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c9fbb72b
authored
Mar 01, 2003
tarafından
Guido van Rossum
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Added implementation notes for [re]set_exc_info().
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Python/ceval.c
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c9fbb72b
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@@ -2616,6 +2616,67 @@ PyEval_EvalCodeEx(PyCodeObject *co, PyObject *globals, PyObject *locals,
}
/* Implementation notes for set_exc_info() and reset_exc_info():
- Below, 'exc_ZZZ' stands for 'exc_type', 'exc_value' and
'exc_traceback'. These always travel together.
- tstate->curexc_ZZZ is the "hot" exception that is set by
PyErr_SetString(), cleared by PyErr_Clear(), and so on.
- Once an exception is caught by an except clause, it is transferred
from tstate->curexc_ZZZ to tstate->exc_ZZZ, from which sys.exc_info()
can pick it up. This is the primary task of set_exc_info().
- Now let me explain the complicated dance with frame->f_exc_ZZZ.
Long ago, when none of this existed, there were just a few globals:
one set corresponding to the "hot" exception, and one set
corresponding to sys.exc_ZZZ. (Actually, the latter weren't C
globals; they were simply stored as sys.exc_ZZZ. For backwards
compatibility, they still are!) The problem was that in code like
this:
try:
"something that may fail"
except "some exception":
"do something else first"
"print the exception from sys.exc_ZZZ."
if "do something else first" invoked something that raised and caught
an exception, sys.exc_ZZZ were overwritten. That was a frequent
cause of subtle bugs. I fixed this by changing the semantics as
follows:
- Within one frame, sys.exc_ZZZ will hold the last exception caught
*in that frame*.
- But initially, and as long as no exception is caught in a given
frame, sys.exc_ZZZ will hold the last exception caught in the
previous frame (or the frame before that, etc.).
The first bullet fixed the bug in the above example. The second
bullet was for backwards compatibility: it was (and is) common to
have a function that is called when an exception is caught, and to
have that function access the caught exception via sys.exc_ZZZ.
(Example: traceback.print_exc()).
At the same time I fixed the problem that sys.exc_ZZZ weren't
thread-safe, by introducing sys.exc_info() which gets it from tstate;
but that's really a separate improvement.
The reset_exc_info() function in ceval.c restores the tstate->exc_ZZZ
variables to what they were before the current frame was called. The
set_exc_info() function saves them on the frame so that
reset_exc_info() can restore them. The invariant is that
frame->f_exc_ZZZ is NULL iff the current frame never caught an
exception (where "catching" an exception applies only to successful
except clauses); and if the current frame ever caught an exception,
frame->f_exc_ZZZ is the exception that was stored in tstate->exc_ZZZ
at the start of the current frame.
*/
static
void
set_exc_info
(
PyThreadState
*
tstate
,
PyObject
*
type
,
PyObject
*
value
,
PyObject
*
tb
)
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